Systems Made Simple: Solar Inverters Part 1
One of the key systems in a solar energy harvester is a solar inverter. A solar inverter, or any kind of inverter for that matter, will take a Direct Current voltage input and convert it to an Alternating Current output that can be used to power standard appliances and electronics in a home or business. While just about any high power DC source can be used, the largest sector of inverter growth is in renewables, particularly solar applications. This training considers the following design challenges to enable your design:
- Understand which inverter to consider for your solar application
- Learn how an inverter manages to convert DC voltage to AC voltage and why digital controllers are so important
- Learn how to evaluate and select the right digital controller for your design
- How to overcome voltage limitations
- Learn which kinds of communication standards can be used for monitoring their inverters to enable a more connected home or building
Learn more about TI’s solutions for solar inverters at www.ti.com/solarinverters and www.ti.com/microinverters
Resources
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Hi. I am Jayanth Rangaraju, and with me today is Bart Basile from Industrial Systems. Today we welcome you to our new video series called Systems Made Simple. It's been a record year for our solar energy harvesting with about 40 gigawatts of capacity added worldwide. One of the key systems in each of these solar energy harvesters is a solar inverter.
A solar inverter, or any kind of inverter for that matter, will take a direct current voltage input and convert it to an alternating current output that can be used to power standard appliances and electronics in a home or business.
When installed in a home or a business, the solar inverter can connect to the electricity grid to offset consumption or, in some cases, even provide energy back to the utility. To do this, it must be able to synchronize its AC output with the grid voltage and comply with certain safety requirements, such as surge shutting of the AC output when the grid voltage disappears.
Just like everything else, inverters come in various forms and sizes, such as microinverters, string inverters, center inverters, and so on. Bart, can you help our viewers understand which inverter they should consider for their solar application?
Traditionally, a series of solar panels are connected to a string inverter. These inverters take approximately 600 volts DC input, which equates to a few kilowatts of solar capacity. An inverter for a full string will need to be sized appropriately, but it centralizes the conversion and makes the overall solar system cheaper to install when designed correctly.
A newer technology emerging is the micro solar inverter. These are sized to match a single panel or about 200 to 300 watts. By distributing the inversion process, the solar array can accommodate much more complex rooftops and enable smaller arrays to be installed that wouldn't typically reach the input voltages of the string inverter. Here is a system block diagram of the solar inverter. The key subsystems are the digital controller, gate drivers, isolation, and communication.
That is great. The key functionality of an inverter is to convert DC that we harvest out of a solar panel into AC. Bart, can you briefly explain to our viewers how an inverter manages to convert DC voltage to AC voltage?
As you can imagine, converting from DC to AC isn't a trivial task in the electronics world. An inverter first needs to convert the input voltage to a steady, usable DC. This is usually in the hundreds of volts so that it can be used effectively to create the peak voltages of the AC wave. These high voltage differentials create their own problems in the inverter design.
Modern inverters use a digitally controlled dynamic buck converter to generate the waveform. This way, we can match the grid frequency and tune the output of our filters more effectively for less losses. The digital stage has a myriad of sensor inputs as feedback into the controller to ensure the proper voltages are being generated and extracting the maximum amount of power from the DC source.
OK. These digital converters seems pretty important in inverters. How do our customers choose the right controller for their solar inverter design?
That's right. The controller choice is vital an AC inverters and serves as the heart of the system. This controller is typically a DSP that is capable of processing all of the signal inputs and running at a high enough speed to maintain tight control over the ever-changing voltage output of the buck converters.
Since most of the control system is borrowed from digital power architectures, similar devices are used-- features that really help our integrated high speed ADCs and PWM outputs, onboard coprocessors, and robust physical architectures. Here at TI, we have a dedicated group of digital power experts who can tell us more about choosing the right DSP for the job.
In most electronics systems, anything over 48 volts is considered high voltage. What kind of technologies are required to operate at hundreds of volts?
Overcoming voltage limitations is a huge problem in any power electronic design. In order to sense and control these voltages, we use capacitive isolation devices here at TI. These devices allow high frequency signals to cross power boundaries but block the high voltage DC. This isolation technology has a long life expectancy and low electromagnetic emissions, making it well-suited for industrial applications. In the inverters, we also use isolated power supplies so that we can power the electronics on the other side of the isolation boundary effectively and high voltage FETs and IGBTs to control the power paths.
With multiple high voltage domains that need control across these isolation barriers, the power architecture can become fairly complex. Luckily, we have the devices to make this easy here with not only the silicon and power experts, but tools like WebBench and TI Designs to help out.
What kinds of considerations do our customers take into account with these high-powered devices? These inverters can output power in the kilowatt range. Even at 120 volts, that's a lot of current.
For the power path control, we use MOSFETs and IGBTs. These devices are designed to be switched very high voltages and currents, which make them a great fit in the digital buck converters in an inverter. The real key with using these devices is driving them properly. The input acts as a capacitor that must be charged and discharged each time the FET is switched. When switching these devices at the high speeds required by the inverter, several amps can be required to properly drive them.
If they aren't switched fast enough, the conversion stage can suffer huge efficiency losses. To do this properly, dedicated drivers are used, which translate the digital PWM from the controller to the current required by the FET. As inverter technology continues to evolve, higher switching frequencies are being utilized in order to reduce the size of the magnetic components. This is leading to even faster charge times and continued improvements in FETs and their drivers.
With connected homes being such a big area of interest these days, what kind of communication standards can our customers choose for monitoring their inverters?
A lot of people are interested in seeing live production statistics of their in-home solar installation in order to ensure that their investment is paying off. Adding Wi-Fi or ethernet to an inverter isnt' that hard of a task with the embedded networking that is in many of our microcontrollers. Once connected to your home network, getting the data onto the cloud for the user to view anywhere is just a step away. Additionally, the communication system can be used to monitor the system and alert the owner to any potential upcoming maintenance.
Well, luckily, we can also elaborate some of the communication technology that has been powering this market for years now. Low-power wireless connectivity standards like ZigBee or 6LoWPAN can help get to those hard to reach places. Be sure to check out ti.com/solarinverter for solar inverter system solutions. Thanks, Bart, for making solar.
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